Antinociceptive effects of morphine, D-Ala 2-Met-Enkephalin, D-Ala 2-Met-Enkephalinamide, D-Ala 2-Ala 2-Leu-Enkephalin, and beta-Endorphin are studied following cannulation of these substances into the midbrain ventral central gray. Aversive stimuli contrasted are thermal nociceptive heat applied to face, feet, and tail, as opposed to central stimulation of dorsal central gray and trigeminal complex. Thus far, D-Ala 2-Met-Enkephalin is observed to behave like morphine in that both opiates produce profound analgesia for peripheral heat stimulation but do not elevate the aversive threshold for aversive brain stimulation in the dorsal central gray of midbrain. It remains to test drug effects with central stimulation at the trigeminal complex and thalamic ventral basal complex, and also to complete studies with the other named opiates.